1. Field of the Invention
This invention relates to a control system, and more particularly, to a control system required to have a good robustness to disturbances, such as a control system for a motor.
2. Description of the Prior Art
In conventional control systems, a PI controller (Proportional-Integral controller) has been widely used in order to improve the robustness to disturbances, or to suppress the influences of disturbances. Recently, the PI controller can provide a digital control using a microprocessor. For example, a digital PI controller using a microprocessor for controlling the speed of a DC motor is disclosed in "Digital Control Systems", chapter 14, section 14.4, pp 689-694, written by Benjamin C. Kou and published as Holt-Saunders International Editions.
The PI controller, either analog or digital, can improve the robustness to disturbances more at lower frequencies. However, the recent requirement to improve the robustness to disturbances has become more and more severe in some applications. For example, a control system for controlling the speed of a capstan motor in a video tape recorder requires a highly improved robustness to torque disturbances, because the size and the inertia of the capstan motor have been greatly minimized in recent years.